The present invention relates to an electric machine with a damping device. The electric machine is an AC machine or a three-phase machine, and can be implemented as a rotary machine or as a linear motor.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
Electric machines, in particular synchronous motors, are frequently constructed by inserting specially formed winding wires in rotor or stator grooves. The winding wires are connected at the winding head to form winding strands. A winding strand can have several coils, depending on the number of poles. In three-phase machines, each phase has at least one winding strands, with the winding strands being connected at a star point. An exemplary a three-phase synchronous motor with four poles has three winding strands, each having four coils that are connected at the star point. The input voltages of the three-phase current are connected to the terminals of corresponding winding strands located on the other side of the star point.
The windings of linear motors and more recently also of synchronous motors, which must have a high torque, are implemented in exciter coil technology. In other words, the magnetic cores are made of laminated metal sheets with teeth that operate as pole cores. The prefabricated coils are placed on the individual pole cores and suitably connected. The prefabricated coils are made of a support or coil bobbin which is fabricated of an electrically insulating material, and include insulated copper wires which are typically applied by machines. Optionally, the support can also be removed after the winding has been completed, if the windings themselves have enough stability.
The electric machine can receive electric power from a converter, which can be an inverter. The converter can include, for example, an inverter, a DC link circuit and a rectifier.
During operation of electric motors powered by pulse converters, the switching operations of the converter can cause voltage peaks between the motor winding and the grounded motor body. The motor winding operates as a reactant network conductor that is capacitively coupled to ground via the winding insulation. The voltage peaks are particularly large at the end of the reactant network conductor, which is frequently the star point of the winding. The reactant network conductor is also characterized by resonances and a resonant frequency. If all input terminals of the motor winding are simultaneously switched hard in one direction, then an electric oscillation with a characteristic frequency is produced between the motor winding and ground potential, which decays only slowly due to the weak natural damping. If the switching operations on the input terminals are periodic and their fundamental frequency or harmonics are close to the characteristic frequencies of the reactant network conductor of the motor winding, then the voltage peaks can become so large as to diminish the useful life of the insulation. Voltage peaks due to switching operations are therefore undesirable.
Breakdowns at the star point of three-phase motors constructed with the exciter coiled technique can occur in particular in large converter installations.
Similar problems can occur with electric machines suffering from transient overvoltages. The overvoltages should hence be limited to prevent breakdowns. For example, as disclosed in German patent publication no. DE 38 26 282, a voltage-dependent metal oxide resistor can be connected in parallel with a coil for limiting the overvoltage. German patent publication no. DE 28 34 378 discloses short-circuited winding sections for damping transverse fields. Likewise, German patent publication no. DE 24 33 618 discloses a method for damping transient overvoltages in a synchronous machine by applying transverse field damper rods.
European patent publication no. EP 0 117 764 describes a method for suppressing overvoltages produced by resonance phenomena through application of ferroelectric isolators disposed between the coil windings. European patent publication no. EP 0 681 361 addresses the problem associated with higher harmonic oscillations which can occur in converters and rectifiers employing power thyristors. The damper winding is then connected with capacitors to form resonant circuits. The resonant circuits have a resonance frequency which is 6n times greater than a fundamental frequency of synchronous machine. In this way, higher harmonic oscillation of the fundamental can be absorbed.
Nevertheless, there remain problems of breakdowns in the star point of a synchronous motor produced with exciter coil technique. German patent publication no. DE 100 49 817 discloses an induction device as well as a method for operating electric machines that reduce the risk of a breakdown in the star point. This published patent application discloses an induction device with a winding arrangement having a winding start and a winding end for inducing a magnetic field in a magnetizable core and a lossy magnetizable device. The winding start and the winding end of the winding assembly are routed through or around the lossy magnetizable device in such a way that a magnetic flux is excited in the lossy magnetizable device.
A drawback of conventional devices is the need for additional installation space and added costs.
It would therefore be desirable and advantageous to provide an improved damping device for an electric machine, which is compact and can be manufactured cost-effectively.